The binding mechanism for colchicine and tubulin was elucidated from kinetic studies. The mechanism is a two step binding process consisting of an initial rapid equilibrium followed by a rate limiting conformational step. Rate constants for all steps were determined. Modification of tubulin by membrane components change the interaction of colchicine with tubulin. The nature of the modification is not yet known. In vitro polymerization in crude supernatant was inhibited by sodium fluoride, microsomal fraction, GMP, AMP, and to a lesser extent by CMP. These effects were not observed with purified tubulin. BMP does not inhibit the ATP induced polymerization. The inhibition is not due to chelation of Mg ion nor to competition for GTP at the exchangeable nucleotide site on tubulin. BIBLIOGRAPHIC REFERENCES: Garland, D.L. (1977) Fed. Proc. 36, 899. Evidence for Colchicine Induced Protein Conformational Change in Tubulin.